Dynamoelectric machines are machines that generate electric power or use electric power. Common types of dynamoelectric machines are alternators, generators, and electric motors.
Electric motors are used in a wide variety of applications involving power tools such as drills, saws, sanding and grinding devices, and yard tools such as edgers and trimmers, just to name a few such tools. These devices all make use of electric motors having an armature and a field, such as a stator.
FIG. 1 shows a typical prior art stator 100 for an electric motor. Stator 100 is formed from a lamination stack 102, having a length L, around which a plurality of windings of magnet wires 104 are wound to form field coils 114. Lamination stack 102 is formed by stacking together an appropriate number of individual laminations 108 and welding them together. The individual laminations 108 are typically made by stamping them from steel. To do so, loose laminations 108 are loaded in a stacker. The stacker picks up the appropriate number of laminations 108 and places them in a fixture where they are welded together. The laminations 108 are formed with slots so the resulting lamination stack 102 has slots 110 therein in which the magnet wires 104 are wound. Magnet wires, as that term is commonly understood, are wires of the type conventionally used to wind coils in electric machines, such as armatures and stators. Prior to winding the magnet wires 104, insulating sleeves or insulating slot liners (not shown), such as vulcanized fiber, are placed in the slots 110 and end rings 112 placed on the lamination stack 102. End rings 112 are illustratively made of plastic and formed to include coil forms 116. Field coils 114 are then wound by winding the magnet wires 104 in the slots 110. After the field coils 114 are wound, the end of the magnet wires 104 are appropriately terminated, such as to terminals 118 in a terminal post 120. The magnet wires 104 are then bonded together, such as by the application of heat when bondable magnet wires are used. Bondable magnet wires are magnet wires layered with a heat activated thermoplastic or thermoset polymer adhesive. One type of bondable magnet wires commonly used is wire available under the trade name BONDEZE from Phelps Dodge of Fort Wayne, Ind. Alternatively, the magnet wires 104 may be bonded by a trickle resin process described below. Where the stator 100 will be used in an application that exposes it to a particularly abrasive environment, such as a grinder, an epoxy coating is applied to the field coils 114 for abrasion protection.
As is known, motor output power is a product of motor speed and torque, so one approach to boosting output power is to change the windings of the coils so that the motor runs faster. However, this is often impractical as higher motor speeds puts greater stresses on gears, raises overall vibration levels, and may result in the speed of the tool exceeding rated speeds for associated accessories, such as drill bits, etc.
If any of these limitations exist and the motor speed cannot be increased, an alternative approach is to increase the stack length. There is a general relationship that provides that the motor output power is directly proportional to the product of the length of the stack of the motor's stator and the square of the armature diameter. For example, for a given motor speed the motor output power can be doubled by doubling the stack length or increasing the armature diameter by a factor of √{square root over (2)}.
Increasing the stack length is not practical in many power tool applications because it may negatively impact the tool's ergonomics or increase the overall length of the tool to the point where it is cumbersome to use. For example, a reciprocating saw with an extended or longer body is more difficult to maneuver in tight spaces, such as cutting pipes in walls of buildings, etc.
While increasing armature diameter is an option to increase motor output power, this results in almost a linear increase in the diameter of the field or stator, which may again adversely affect the ergonomics of the tool.